The invention relates to avermectin B2a microcapsule granules. The microcapsule granules include, by weight, 0.1-20% of avermectin B2a, 2-5% of a microcapsule slow-release agent, 1-3% of a dispersant, 1-2% of a wetting agent and 2-10% of water, with the balance being a filler. A preparing method of the granules is also provided. The method includes adding the water and the microcapsule slow-release agent in prepared amounts into a container, fully stirring the mixture until a capsule shell is formed, then adding the dispersant and the wetting agent into the mixture, stirring the mixture, then adding the avermectin B2a that is an active compound, fully stirring the mixture, then performing high-speed shearing for 20 min or above to allow the avermectin B2a to be fully covered with the microcapsule slow-release agent, then adding the product into red sand under stirring, and fully stirring the mixture to obtain granules. The granules have characteristics of a wide spectrum, a high efficiency, a long period of preserving effects, low toxicity and safety.